1. Field of the Invention
The present invention generally relates to a semiconductor ceramic for thermistors (hereinafter referred to as thermistor semiconductor ceramic) which is used as a temperature-compensating element for electronic devices, and more particularly to a thermistor semiconductor ceramic containing zinc oxide and titanium oxide as main components, and to a chip-type thermistor including the thermistor semiconductor ceramic.
2. Description of the Related Art
A temperature-sensitive resistance element such as a positive temperature coefficient thermistor or a negative temperature coefficient thermistor is conventionally used as a temperature-compensating element in electronic devices.
Although a semiconductor oxide or a semiconductor metal is normally used for such a temperature-sensitive resistance element, only temperature-sensitive resistance elements made of a thermistor material having a B constant of about 1,500 K or more are available.
When a temperature-sensitive resistance element having a B constant of less than the above value is required, the above conventional element is connected in parallel to a fixed resistor. The B constant consequently decreases in the combination of the element and the resistor, so that the combination functions as the temperature-compensating element having the desired resistance-temperature characteristic.
However, when the temperature-compensating element is connected in parallel to the fixed resistor, the following problems arise: increasing cost due to the additional components; and difficulty in miniaturizing electronic devices including the element and the resistor due to the amount of space required for the resistor.
In order to solve the above problems, first, improving the known materials of the thermistor has been proposed. However, it is not necessarily easy to decrease the B constant by using known thermistor materials.
Secondly, decreasing the B constant by varying the composition ratio of the thermistor materials has been proposed. This method, however, has a problem in that the reliability of the temperature-compensating element decreases. Also, only by varying the composition ratio of the thermistor material, it is difficult to obtain a thermistor material which provides a series of elements having various B constants in a low range, for example 0 to 1,000 K. A material having sufficient reliability in practical use has not yet been developed.
Accordingly, it is an object of the present invention to provide a semiconductor ceramic which is suitable for thermistors having a B constant in a low range, for example 0 to 1,000 K, and a chip-type thermistor including the thermistor semiconductive ceramic.
In order to achieve the above object, a thermistor semiconductor ceramic contains zinc oxide and titanium oxide which are main components and a predetermined content of manganese.
Adding manganese to the thermistor semiconductor ceramic of which the main components are zinc oxide and titanium oxide (normally a mixed crystal composed of ZnO and Zn2TiO4) varies the resistance-temperature characteristic. The characteristic is controllable in the range of positive temperature coefficient to negative temperature coefficient according to the manganese content, so that thermistor materials which provide a series of thermistor semiconductive ceramics having various B constants are available. Also, the thermistor semiconductor ceramic is suitable for various applications.
When the thermistor semiconductor ceramic contains manganese in the range of about 0.001 to 10 mol %, the resistance-temperature characteristic is reliably controllable so that thermistor semiconductor ceramics having desired characteristics are available.
A thermistor semiconductor ceramic having a positive resistance-temperature characteristic is available when the manganese content is about 0.5 mol % or less, and a thermistor semiconductor ceramic having a negative resistance-temperature characteristic is available when the manganese content is about 0.5 mol % or more.
Controlling the manganese content provides a series of thermistor semiconductor ceramics having various B constants in a low and wide range of 0 to 1000 K.
When the thermistor semiconductor ceramic contains a predetermined content of nickel, the resistivity is controllable so that a series of thermistor semiconductor ceramics having various resistivities are available.
When the nickel content is preferably in the range of about 0.1 to 20 mol %, a series of thermistor semiconductor ceramics having a resistivity in the range of 1 to 1,000 xcexa9xc2x7cm, which is useful in practice, are available.
A chip-type thermistor includes a thermistor element comprising the semiconductor ceramic and electrodes provided on the thermistor element.
When the electrodes are provided on the thermistor element comprising the semiconductor ceramic, a chip-type thermistor having a B constant in a low range of 0 to 1000 K, for example, can be provided.